The present invention relates generally to the drying of wettable webs, and more particularly to a machine for carrying out such drying.
The invention is concerned in particular with drying of wet paper webs in a paper-making machine, and will hereafter be described with reference to such an application. It should be understood, however, that the invention can also be successfully employed in drying of other webs from which moisture must be removed.
It is known that in paper-making machines the newly produced paper web is wet and must be dried, both to improve its strength and to prepare it for subsequent additional processing steps. For this purpose, drying arrangements are known whose purpose it is to withdraw moisture from the web. Thus, German Patent No. 1,911,653 (corresponding to U.S. application Ser. No. 712,260 now U.S. Pat. No. 3,503,139) discloses an arrangement on which the web to be dried is supported on an endless traveling band of felt which carries it through the drying arrangement. The drying rollers comprise a series of upper rollers which are hollow and have permeable walls, and whose exterior is connected to a source of suction in order to withdraw moisture from the web by drawing away from the interior of the rollers. Located in a lower second plane are rollers which are heated in order to expel residual moisture from the web. In this prior-art construction the side of the felt web on which the wet paper web is supported faces away from the upper suction rollers as it travels around the same, so that it is the other side--the one which does not support the wet paper web--which is in direct contact with the circumferential walls of the suction rollers. Conversely, when the felt band and the wet web travel around the heated lower rollers, the side of the felt band on which the wet paper web is supported faces towards the heated rollers so that the wet paper web is in direct contact with the surfaces of the heated rollers against which it is pressed by the felt band. It is desired that during its travel around both types of rollers the paper web is subjected as much as possible to identical temperatures, and the rollers which form contact heaters and are engaged by the paper web directly are to be heated to a lesser extent than the upper suction rollers in order to avoid damage to the paper web.
It has been found that this arrangement is not fully satisfactory. In particular, optimum drying conditions cannot be obtained with this prior-art arrangement, for various reasons of which one is the fact that the wet paper web is not in direct contact with the heated suction rollers located at the upper level or in the upper plane, so that the heat from these rollers must first penetrate through the felt carrier band before it can act upon the paper web. Again, when the felt band and the wet web travel around the heated lower rollers in contact therewith, the moisture which is expelled from the web is received in the felt band and, when the band and the web subsequently move into contact with the next one of the rollers at the upper level, this moisture is reabsorbed into the paper web from the felt band. On the other hand, it is not possible to eliminate the felt band entirely because the paper web is not yet sufficiently strong to be self-supporting, especially at the relatively high speeds at which it is required to travel to and past the different rollers.